From the blog (?) notes.unwieldy.net:
The average New York City taxi cab driver makes $90,747 in revenue per year. There are roughly 13,267 cabs in the city. In 2007, NYC forced cab drivers to begin taking credit cards, which involved installing a touch screen system for payment.
During payment, the user is presented with three default buttons for tipping: 20%, 25%, and 30%. When cabs were cash only, the average tip was roughly 10%. After the introduction of this system, the tip percentage jumped to 22%.
He calculates that the tip nudge increased cab revenues by $144,146,165 per year.